The present invention relates to a thermal barrier coating applied to the surface of a superalloy article e.g. a gas turbine engine turbine blade, and to a method of applying the thermal barrier coating. The invention particularly relates to ceramic thermal barrier coatings.
The constant demand for increased operating temperature in gas turbine engines was initially met by air cooling of the turbine blades and turbine vanes and the development of superalloys from which to manufacture the turbine blades and turbine vanes, both of which extended their service lives. Further temperature increases necessitated the development of ceramic coating materials with which to insulate the turbine blades and turbine vanes from the heat contained in the gases discharged from the combustion chambers, again the operating lives of the turbine blades and turbine vanes was extended.
It is known in the prior art to apply these ceramic coating materials by the thermal or plasma spray process onto a suitable bond coating, for example a MCrAlY alloy bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by the physical vapor deposition process onto a suitable bond coating which has an alumina interface layer, for example a MCrAlY alloy bond coating or a diffusion aluminide bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by plasma spraying or physical vapor deposition processes onto an oxide layer on the metallic substrate.
The ceramic thermal barrier coatings deposited by the physical vapor deposition process has benefits over the ceramic thermal barrier coating deposited by plasma spraying. The main benefit is improved thermal shock resistance due to the columnar structure of the ceramic thermal barrier coating produced by the physical vapor deposition process.
One problem associated with thermal barrier coatings produced by physical vapor deposition is that they suffer from spallation of the ceramic thermal barrier coating. Spallation of the ceramic thermal barrier coating is the loss of the ceramic thermal barrier coating by flaking from the bond coating due to the thermal cycling conditions. Currently it has been found that the ceramic thermal barrier coatings produced by Chromalloy United Kingdom Limited have spalled at the leading edge and trailing edge regions of the turbine blades and turbine vanes or other regions where there is a rapid change in the shape of the component.